Rick, N6RK said:
"...then I found that it barely gets down to 2 MHz with a CB whip."
A short extension and a loading hat from Henry at Texas Bugcatchers
would almost certainly bring it down into the band. Henry recommends
that the separation between the loading hat and the top of the coil be
no more than 2-3 coil diameters.
"...The extension puts the height at almost 15 feet."
I just cut my HF whip down to 13' 6", which is the regulation height
the highway people like to enforce. Before I did this, I trimmed
every tree and filling station canopy I got near. Losing a few feet
at that end of the antenna will make no measurable difference in
performance anyway (unless you move the loading coil and top hat out
near the top of the whip).
"...I am planning to add some base loading to eliminate the top hat
and extension."
For the sake of efficiency, which is going to be poor anyway, I'd
concentrate on the center coil and loading hat. With a 9 or 10-inch
top o.d. hat, the whip above won't have to be more than 4 or 5 feet
long. The rest of the loading ought to be in the center coil,
assuming that you've got just enough inductance at the base to give
you a match to the coax.
One of my ARRL antenna books has software for calculating mobile
antenna characteristics. It's very useful for estimating the effects
changes have on antenna behavior.
See you on 160 mobile in a few weeks.
Brad, KV5V
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